A PET (Positron Emission Tomography) apparatus will be described as an example of nuclear medicine diagnostic apparatus, i.e. ECT (Emission Computed Tomography) apparatus. The PET apparatus is constructed to detect a plurality of γ-rays generated by annihilation of positrons, and to reconstruct a sectional image of a patient only when a plurality of detectors simultaneously detect the γ-rays.
Specifically, a patient is medicated with a radioactive drug including a positron-emitting radionuclide, and detectors consisting of numerous detecting element (e.g. scintillator) groups detect pair annihilation γ-rays of 511KeV released from the patient medicated. And when two detectors detect γ-rays within a definite period of time, they are counted as one pair of annihilation γ-rays detected as a coincidence, and a pair annihilation generating point is determined to exist on a straight line linking the detector pair having detected them. Such coincidence information is accumulated and reconstruction is carried out to obtain a positron-emitting radionuclide distribution image (i.e. a sectional image).
At this time, image resolution of the sectional image is improved by discriminating not only γ-ray detecting positions (γ-ray incident positions) on the detectors but γ-ray detecting positions on the scintillators as more particular γ-ray detecting positions, to increase γ-ray detecting accuracy. So, the number of scintillators is increased to increase discriminating capability. In recent years, in particular, DOI detectors have been developed, which have scintillators laminated also in a depth direction to be capable of discriminating light source positions having caused interaction in the depth direction (DOI: Depth of Interaction).
To discriminate γ-ray incident positions, a two-dimensional position map prepared beforehand is used. The two-dimensional position map is a map showing, in two dimensions, the number of luminescent photons (corresponding to a count value of γ-rays) obtained with light sensors represented by photomultiplier tubes (PMT), as corresponding to incident positions of γ-rays incident on the scintillators. FIG. 10 shows a two-dimensional position map in the case of a DOI detector having four layers of scintillators laminated in the depth direction. The positions indicated by white circles (shown as “◯” in FIG. 10) are scintillators in the first layer (written “1st Layer” in FIG. 10). The positions indicated by white rhombuses are scintillators in the second layer (written “2nd Layer” in FIG. 10). The positions indicated by white double octagons are scintillators in the third layer (written “3rd Layer” in FIG. 10). The positions indicated by white rectangles (shown as “□” in FIG. 10) are scintillators in the fourth layer (written “4th Layer” in FIG. 10). Incident positions of actually incident γ-rays can be discriminated by referring to a look-up table (LUT) having each position in the two-dimensional position map corresponding to each scintillator, and referring to the two-dimensional position map.
Incidentally, where a plurality of scintillators are arranged in three dimensions as in the DOI detector, diffusion is provided by combination of a light reflective material and a light transmissive material, for example, between adjoining scintillators, so that positions do not overlap in the two-dimensional position map. Further, a technique of correcting the two-dimensional position map has been introduced, which carries out a statistical clustering process in order to increase the discriminating capability still further (see Patent Document 1, for example).
In order to prepare a map accurately also when this two-dimensional position map has a large distortion, Inventors herein have proposed a technique of partly preparing a histogram from the map and constructing a map successively from a portion of clear peak toward end portions (see Patent Document 2, for example).
[Patent Document 1]
Unexamined Patent Publication No. 2005-43104
[Patent Document 2]
International Publication No. WO2009-116174